Harris played for the Tennessee Volunteers as a freshman in 2010–11 for coach Bruce Pearl. He played the point forward position, a mix between forward and point guard, in Pearl's offense. A USBWA Freshman All-America second-team selection and one of the nation's most consistent and versatile performers, Harris also was named second-team All-SEC by the league's head coaches and earned a spot on the SEC All-Freshman Team. He ranked fifth among all freshmen in the six major conferences with 15.3 points per game. In addition, his 7.3 rebounds per game ranked sixth among all major-conference freshmen.[1]

On May 9, 2011, Harris declared for the NBA draft, forgoing his final three years of college eligibility.[2]

Harris was selected with the 19th overall pick in the 2011 NBA draft by the Charlotte Bobcats. He was later traded to the Milwaukee Bucks on draft night. On December 10, 2011, he signed his rookie scale contract with the Bucks.[3] Harris made his NBA debut on January 7, 2012 against the Los Angeles Clippers before tallying a team-high 15 points a night later against the Phoenix Suns on January 8. He appeared in 42 games during his rookie season (nine starts), averaging 5.0 points, 2.4 rebounds and 0.5 assists in 11.4 minutes per game.

On October 24, 2012, the Bucks exercised their third-year team option on Harris' rookie scale contract, extending the contract through the 2013–14 season.[4]

On February 21, 2013, Harris was traded, along with Doron Lamb and Beno Udrih, to the Orlando Magic in exchange for J. J. Redick, Gustavo Ayón and Ish Smith.[5] Harris' playing time saw a notable increase in Orlando, and his stats followed suit. Harris more than tripled his points per game average, and more than quadrupled his rebounds, assists and blocks per game.

On October 26, 2013, the Magic exercised their fourth-year team option on Harris' rookie scale contract, extending the contract through the 2014–15 season.[6]

On January 24, 2014, Harris scored 28 points and grabbed a career-high 20 rebounds in a 114-105 win over the Los Angeles Lakers.[7] On March 2, 2014, he scored a career-high 31 points in a 92-81 win over the Philadelphia 76ers.[8]

Harris wears number 12 to tribute his close friend and former teammate Morgan Childs, who died at age 17 due to leukemia. Although he was unable to wear 12 during his time with the Milwaukee Bucks, Harris switched back after he was traded to the Orlando Magic during the 2012–13 season.[12]